Generally, spot welding is used for welding together members of the same metal. Spot welding will contribute greatly to weight reduction of material, provided that members respectively of different metals, such as an aluminum member and an iron-base member can be welded together by spot welding. The term “aluminum” is used for designating pure aluminum and aluminum alloys, and the term “steel” will be used instead of “iron-base” in this specification.
A brittle intermetallic compound is likely to be produced in the joint of a steel material and an aluminum material when the steel and the aluminum material are welded together and consequently, it has been very difficult to form a reliable, very strong joint, namely, a joint having high weld strength. Therefore, bolts and rivets have been used for welding together members respectively of different metals. However, joints formed by using bolts or rivets have problems in reliability, air tightness and cost.
Many studies have been made of spot welding methods of fabricating welded materials each formed by welding together materials respectively of different qualities. Methods that insert an aluminum-steel clad material or a steel pad between a steel material and an aluminum material are proposed in Patent documents 1, 2, 3 and 4. Methods that plate a steel material with a metal having a low melting point or insert a sheet of a metal having a low melting point between a steel material and an aluminum material are proposed in Patent documents 5, 6 and 7. A method proposed in patent document 8 sandwiches insulating particles between a steel material and an aluminum material. A method proposed in Patent document 9 forms irregularities in material. A method proposed in Patent document 10 forms nuggets in the interface between a steel plate and an aluminum alloy plate so as to be biased toward the positive pole, namely, the aluminum alloy plate, by the agency of a polarity effect.
[Patent document 1]JP-A No. H6-63763[Patent document 2]JP-A No. H7-178563[Patent document 3]JP-A No. H4-55066[Patent document 4]JP-A No. H7-328774[Patent document 5]JP-A No. H4-251676[Patent document 6]JP-A No. H7-24581[Patent document 7]JP-A No. H4-143083[Patent document 8]JP-A No. H5-228643[Patent document 9]JP-A No. H9-174249[Patent document 10]JP-A No. H5-111776